Certain types of broadband multichannel radio transmitter equipment is finding increasing use in wireless communication systems such as Cellular, Mobile Telephone and Personal Communication Services systems. This equipment is capable of processing many Radio Frequency (RF)carrier signals using common radio frequency circuitry. The individual transmitted signals are first formed as digital baseband signals such as by a bank of Digital Signal Processors (DSPs). The digital baseband signals output by the DSPs are fed in parallel to a digital combiner which provides a composite signal in digital form. In the digital composite signal, the individual channel signals are each translated to a different digital intermediate frequency, typically as equally spaced digital carrier signals. The digital composite signal is then fed to a digital-to-analog converter, and the resulting multicarrier analog waveform is then fed to broadband Radio Frequency (RF) equipment.
While broadband transmission of many radio channels in parallel through the use of sophisticated digital signal processing techniques provides an advantage of compact size and low price, this comes at the cost of more sophisticated expensive Radio Frequency (RF) equipment. One such system component is the broadband transceiver that transmit the multicarrier waveform as a suitable radio signal. Since the broadband transceiver is transmitting many channels in parallel, it must operate as linearly as possible to avoid adjacent channel and/or harmonic distortion in the resulting transmitted waveform. For example, certain digital wireless protocols such as the Global System for Mobile Communications (GSM) specify that the intermodulation distortion between adjacent channels must be such that a dynamic range of 70 decibels (dB) or more is available.
It has been found that although this approach is particularly advantageous in a multiuser wireless communication system where multiple signals must be transmitted at the same time such as at the base station location, the required digital-to-analog conversion places a limitation on the overall system performance. In particular, the digital-to-analog conversion introduces non-linearities that are manifested as spurious sidelobes. These spurious sidelobes limit the effective dynamic range of the radio transmitter.
Unfortunately, this limitation becomes significant as the number of desired carrier frequencies increases. At a point, it even becomes necessary to consider reverting to conventional types of single channel radio equipment in order to meet the required specification for spurious-free intermodulation distortion.
One approach to increasing spurious dynamic range is to add additional bits to the digital-to-analog converter. It is also possible to modify the analog amplifying circuitry to provide better linearity. However, each of these implementations require modification to a digital-to-analog subassembly design which in turn implies a much higher system cost.
What is needed is a way to increase the overall system performance by reducing the sidelobes produced in the digital-to-analog conversion process without unnecessarily complicating the digital signal processing hardware and without modifying the broadband analog radio equipment.